Thermally sensitive electrical switch

ABSTRACT

A thermally sensitive electrical switch comprises a body having a chamber therein occupied at one end by a thermally expansible and contractile material and at the other end by a resilient, deformable, elastomeric switching pad operable in response to the application of compressive force thereto to establish an electrically conductive path between electrical conductors. A deformable force transmitting member preferably is interposed between the thermally expansible material and the switching pad to compress the latter and to assist in decompressing the pad upon contraction of the expansible material.

United States Patent Du Rocher et al.

[ 1 Oct. 1, 1974 THERMALLY SENSITIVE ELECTRICAL SWITCH [75] Inventors:Gideon A. Du Rocher, Mt. Clemens; Gerald L. McClure, Warren, both ofMich.

[731 Assignee: Essex International, Inc., Fort Wayne, Ind.

[22] Filed: Mar. 7, 1973 [211 App]. No.: 339,000

[52] US. Cl. 337/382, 338/114 [51] Int. Cl. H0lh 37/46 [58] Field ofSearch 337/382, 394, 393, 320;

[56] References Cited UNITED STATES PATENTS 2,744,981 5/1956 Spears338/114 3,212,337 10/1965 McCarrick 337/394 3,509,296 4/1970 Harshman eta1 338/114 Primary Examiner-Harold Broome Attorney, Agent, orFirm-Learman & McCulloch 5 7 ABSTRACT A thermally sensitive electricalswitch comprises a body having a chamber therein occupied at one end bya thermally expansible and contractile material and at the other end bya resilient, deformable, elastomeric switching pad operable in responseto the app1ication of compressive force thereto to establish anelectrically conductive path between electrical conduct ors. Adeformable force transmitting member preferably is interposed betweenthe thermally expansible material and the switching pad to compress thelatter and to assist in decompressing the pad upon contraction of theexpansible material.

16 Claims, 5 Drawing Figures l THERMALLY SENSITIVE ELECTRICAL SWITCH Theinvention disclosed herein relates to a thermally sensitive switchoperable in response to a predetermined rise in temperature to actuatean electrical signal. A switch of the kind with which the invention isconcerned has many applications one of which is that of sensing thetemperature of an automotive engine coolant or transmission fluid andactuating a warning signal in the event of overheating.

Switches of the kind to which the invention relates have been proposedheretofore, but not all of them have been altogether satisfactory for anumber of reasons. For example, some of the known switches haveincorporated one or more bimetallic parts which are responsive tochanges intemperature to energize and deenergize a signaling device.Such bimetallic members are expensive to manufacture and require carefulcalibration if they are to function reliably. The calibration procedureis time consuming, requires considerable skill, and therefore isexpensive.

Others of the known switches have included temperature sensitive probesmounted on a vehicle engine or the like and switch contacts mounted at azone remote from the probes. Such switches thus require a multiplicityof separate parts, together with their mounting means, and electricalconnections between the probes and contacts thereby adding to theexpense of manufacture and installation.

Although others of the known switches have overcome the diffcultiesreferred to above by providing switch contacts carried by spring armslocated within the probe and utilizing a thermally expansible andcontractile substance to effect movements of the springmounted contacts,such switches also require calibration. in addition, such contacts aresubject to pitting and corrosion due to arcing, thereby adverselyaffecting the reliability and longevity of such switches.

An object of this invention is to provide a thermally sensitive switchwhich overcomes the disadvantages of switches used heretofore forsimilar purposes.

Another object of the invention is to provide a thermally sensitiveswitchwhich requires little or no calibration and which overcomes thedisadvantages normally associated with arcing between contacts.

A further object of the invention is to provide a switch of thecharacter described which is economical to manufacture and install.

Other objects and advantages of the invention will be pointed outspecifically or will become apparent from the following description whenit is considered in conjunction with the appended claims and theaccompanying drawings, in which:

FIG. 1 is an elevational view of a switch constructed in accordance withone embodiment of the invention;

FIG. 2 is a vertical sectional view taken on the line 2-2 of FIG. 1 andillustrating the switch in its open or non-conductive condition;

FIG. 3 is a view similar to FIG. 2, but illustrating the switch in itsclosed or conductive condition;

FIG. 4 is a vertical sectional view of another embodiment of theinvention and illustrating the switch in its open or non-conductivecondition; and

HO. 5 is a view similar toFIG. 4, but illustrating the switch in itsconductive condition.

A switch constructed in accordance with the embodiment of theinvention-shown in FIGS. 1 3 comprises a body 1 formed of electricallyconductive metal and having a cylindrical casing 2. One end 3 of thecasing tapers and is externally threaded as at 4 so as to be threadedinto an opening formed in the block (not shown) of a conventionalinternal combustion engine, for example. The opposite end of the casing2 has a flange 5 provided with flats 6 to facilitate the fitting of thebody 2 to the cylinder head.

The body 1 defines a multiple part chamber 7 which extends axially ofthe casing 2. Extending inwardly from the free end of the casing 2 thechamber has a first portion 8 of greatest cross-sectional area whichcommunicates with an intermediate, tapering throat portion 9 which, inturn, communicates with a cylindrical portion 10 of considerably lesscross-sectional area than that of the chamber portion 8. The chamberportion 10 communicates with a cylindrical chamber portion 11 whichextends to the opposite end of the body 1. An upstanding flange 12surrounds the chamber portion 11 at the mouth thereof.

Fitted into the chamber portion 11 is a rigid insulator block 13 throughwhich extends an electrical conductor or electrode 14 that is connectedby wiring via a signal lamp L to a battery B or other electrical energysource. The electrode 14 has a reduced neck 15 between its ends toprevent relative axial movement of the electrode and the insulator 13,and the latter is prevented from axial movement relative to the body 1by a shoulder 16 at the juncture of the chamber portions 10 and 11 andbyrpeening the flange 12 over the insulator 13 as is'shown clearly inH68. .2 and 3. The construction and arrangement of the parts thus fardescribed are such that the inner end of the electrode 14 communicateswith the chamber portion 10.

The body 1 includes a cup-shaped member 17 formed of a heat conductivemetal having a preferably flat bottom wall 18 and an upstanding sidewall 19. The diameter of the side wall 19 is enlarged between its endsto form a shoulder 20, the enlarged diameter of the side wall being suchas to enable the member 17 to fit snugly within the chamber portion 8.The member 17 is maintained in the chamber portion 8 by a shoulder 21adjacent the inner end of the chamber 8 and by a peened over flange 22on the casing 2.

The member 17 contains a quantity of thermally expansible andcontractile material which expands volumetrically in response to a risein its temperature. Preferably, the material 23 is one whose expansionis relatively insignificant until a predetermined, critical temperatureis reached, whereupon the material expands quite rapidly. One knownthermally expansible and contractile material having the desiredcharacteristics is a microcrystaline wax (either with or withoutmetallic particles) manufactured by Vernay Laboratories, Inc. Thetemperature at which such wax material liquifies and expands rapidlyvaries in accordance with the specific formulation of the wax and suchwaxes having greatly different expansion temperatures may be obtained byspecifying the expansion temperature desired. For example, a waxmaterial adapted for use in a device to sense engine coolant temperaturemay have a critical or expansion temperature of about 200 F. fornon-pressurized coolant systems or of about 220 F. for pressurizedsystems. Waxes currently available have critical temperatures as high as270 F.

Between the expansible material 23 and the electrode 14 is a forcetransmitting member or diaphragm 24 formed of electrically insulating,resiliently deformable material such as silicone rubber. The member 24comprises a body 25 of generally frustoconical configuration having aperipheral flange 26 provided with an annular groove in its lowersurface. The free end of the cup wall 19 has an endless rib 27 whichfits into the groove and forms a peripheral seal around the open end ofthe cup member 17. The taper of the body 25 corresponds to the taper ofthe chamber throat 9 and the body 25 terminates in a short, cylindricalfoot portion 28 which extends a short distance into the chamber portion10.

Interposed between the force transmitting member 24 and the electrode 14is a molded switching member 29 comprising a resilient, deformable padof elastomeric, non-conductive material such as silicone rubberthroughout which is dispersed a plurality of electrically conductiveparticles. The particles preferably comprise copper or other base metalspheres coated with a noble metal, such as silver, which has a lowelectrical resistance and which, if it oxidizes, produces anelectrically conductive oxide. v

The cross-sectional area of the switching member 29, when it is notsubjected to compressive force, preferably is less than thecross-sectional area of the chamber portion so as to provide a space 30between the member 29 and the wall of the chamber 10 into which themember 29 may expand when it is subjected to compressive force.

The compresive force to which the member 29 must be subjected to convertit from non-conductive condition to conductive condition depends uponseveral factors, such as the thickness and durometer hardness of theelastomer, the quantity and diameter of the metal particles, and thepressure under which the member 29 originally was molded. These factorsare described in detail in co-pending application Ser. No. 857,941,filed Sept. 15, 1969, and to which reference may be had for a morethorough discussion. Briefly, however, the compressive force required toconvert the member 29 from non-conductive to conductive condition isdirectly proportional to the thickness and hardness of the pad and isinversely proportional to the size and quantity of the particlescontained in the pad. Thus, a given switching member may be renderedconductive in response to light or heavy compressive forces, dependingupon the operating characteristics desired.

In the embodiment of the invention shown in FIGS. 1 3, the switchingmember 29 normally is not conductive, but is rendered electricallyconductive when it is subjected to an axially compressive forcesufficient to deform it radially outwardly into engagement with the wallof the chamber portion 10. In this condition of the pad, a sufficientnumber of the electrically conductive particles contained in the padwill be moved into engagement with one another to form one or moretrains of engaged particles bridging the space between the electrode andthe wall of the casing 2, thereby establishing an electricallyconductive path between the electrode 14 and that portion of the casing2 adjacent the switch member 29. The resistance of the current pathcorresponds to the resistance of the particles.

To condition the apparatus described this far for operation, the body 1may be threaded into an opening formed in the block of an engine so asto locate the cup member 17 in a position to sense the temperature ofthe engine coolant, for example. In this position of the body, thecasing 2 is electrically grounded. Until such time as the temperaturesensed by the material 23 rises to the predetermined, criticaltemperature, the switch member 29 remains relatively uncompressed and nocurrent path exists through the member 29. When the temperature rises tothe critical temperature of the material 23, however, it liquifies andsuddenly expands in volume and applies a force on the member 24displacing the foot 28 toward the member 29 and compressing the latterso as to render it electrically conductive and expand it radially intoengagement with the wall of the casing 2. The wall of the chamberportion 10 guides the foot 28 during movement of the latter.

The chamber throat 9 is formed to have an included angle approaching 90,as a result of which a portion of the body 25 is partially extruded intothe chamber portion 10 and is simultaneously compressed by the wall ofthe throat. Significant spring energy thus is stored in the member 24when it effects compression of the member 29.

When the temperature to which the expanded wax material 23 falls belowthe critical temperature, the material 23 solidifies and contractsthereby relieving the force applied on the member 24. The spring energystored in the member 24 then effects sufficient withdrawal of the foot28 from the chamber 10 to enable the switch member 29 to be decompressedand reassume its non-conductive condition.

The size and number of the discrete conductive particles containedwithin the switching member 29 will vary in accordance with the value ofthe current which must be accommodated by the particles. Sphericalparticles ranging in size from 0.003 inch to 0.008 inch in diameter andconstituting between about 93 weight percent of the pad have been foundto be satisfactory to accommodate current values encountered inautomotive vehicles. Larger size particles may be employed in switchesadapted for use in circuits having current values greater than thoseencountered in vehicles.

Although arcing may occur between adjacent particles upon the making orbreaking of a current path through the switching member 29, therebyresulting in pitting or even destruction of one or more of suchparticles, the pad contains so many particles that particles other thanthose which may be damaged by arcing will form a conductive trainthrough the pad. As a consequence, the useful life of the switchingmember 29 greatly exceeds that of conventional contacts which engage anddisengage each other repetitively.

The quantity of wax material 23 contained in the cup 17 and the extentof compression required to convert the switching member from itsnon-conductive condition to its conductive condition are such that theswitch is rendered conductive in response to a rise in temperature ofthe material 23 within only a few degrees of the critical temperature ofthe wax. Thus, calibration of the apparatus normally is not requiredinasmuch as it is the critical temperature of the wax which triggersoperation of the switch. If it is desired to assure operation of theswitch at the critical wax temperature, however, this may beaccomplished by indenting the base 18 of the cup 17, as shown in dottedlines in FIG. 3, so as to lightly preload the diaphragm 24.

The embodiment of the invention disclosed in FIGS. 4 and 5 is similar inmany respects to the previously described embodiment and differs fromthe latter primarily in that the body itself forms no part of theelectrical current path. The modified embodiment has a body 33comprising a tapered, externally threaded metal casing shank 34terminating at one end in a heat conductive probe 35 adapted to sensechanges in temperature. The other end of the casing shank 34 terminatesin an enlarged sleeve 36 having wrench accommodating flats (not shown)on its outer surface. The sleeve 36 defines a cavity 37 whichcommunicates with a chamber 38 formed in the shank 34. Between thecavity 37 and the chamber 38 is a shoulder having an annular groove 39therein.

Fitted into the cavity 37 is an insulating block 40 forming part of thebody 33 and being retained in the cavity 37 by a peened over flange 41carried by the sleeve 36. Adjacent the inner end of the block 40 is achamber 42 which communicates with the chamber 38 via a tapered throat43 similar to the throat 9.

Extending into the block 40 and communicating with the chamber 42 is apair of spaced apart, electrically conductive electrodes 44, 45, theinner ends of the electrodes preferably being cut away on arcscorresponding to the curvature of the chamber 42 so that the electrodesconstitute portions of the wall of the chamber.

Occupying the chamber 42 is a switching member 46 similar in allrespects to the switching member 29 and having a cross-sectional arealess than that of the chamber 42. The switching member is straddled bythe electrodes 44 and 45.

A thermally expansible and contractile wax material 47, similar to thematerial 23, occupies the chamber 38. Between the switching member 46and the material 47 is a force transmitting member or diaphragm 48similar to the member 24 and having a body 49 provided with aperipheral, annularly grooved flange 50 which interfits with the groove39 to seal the chamber 38. The body 49 tapers complementally to thetaper of the throat 43 and terminates in a foot 51 which projects ashort distance into the chamber 42 to fonn a seat for the switchingmember 46. A disc 52 of insulating material positioned at the base ofthe cavity 42 forms a bearing surface opposite the foot 51. Theswitching member thus is sandwiched between two insulating members. Thedisc 52 avoids engagement between the switching member and theelectrodes until such time as the switching member has been deformedradially an amount sufficient to bridge the space between theelectrodes.

To condition the apparatus for operation, the body 33 is threaded intoan opening formed in the engine block or other device whose temperatureis to be sensed. The electrode 44 is connected by wiring 53 to a batteryB or other source of electrical energy, and the electrode 45 isconnected by wiring 54 to a grounded lamp L or other signal. Theexpansiblematerial 47 is so formulated as to have a normal, non-expandedvolume until a predetermined, critical temperature is sensed. In thenon-expanded condition of the material 47, the switching member 46 isnon-compressed and is electrically non-conductive.

When the temperature of the material 47 rises to its predetermined,critical temperature it expands suddenly, thereby displacing the foot 51of the force transmitting member 48 into the cavity 42 in the samemanner earlier described. The switching member 46 thus is compressedbetween the foot 51 and the insulating disc 52 so as to render themember 46 electrically conductive and is expanded radially intoengagement with the electrodes 44 and 45. An electrical circuit thus isestablished between the battery B and the signal L.

When the temperature sensed by the material 47 falls below its criticaltemperature, the material contracts, thereby enabling the spring energystored in the stressed force applying member 48 to return the latter toits original condition. The inherent resilience of the switching member46 decompresses it so as to break the circuit to the signal L.

There may be instances in which it is desired to sense temperatures inexcess of the maximum capable of being sensed by presently availablewaxes. For example, it may be desired to operate a signal when thetemperature of a vehicles transmission fluid rises above 280 F. In sucha case, a known eutectic solder having a critical temperature of 280 F.or 281 F. may be substituted for the wax material previously described.Such solder functions very much like the wax in that it undergoes verylittle expansion until its critical temperature is reached whereupon itliquifies and expands suddenly.

The critical temperature at which a device constructed according to theinvention operates can be varied in other ways. For example, the stifferor harder the diaphragm 25 or 51, the greater the resistance exertedthereby to the expansion of the expansible material. As a consequence, ahigher temperature will be required to enable the expansible material todeform the diaphragm and render the switching member conductive. Thestiffness or hardness of the diaphragm may be varied by conventionalmolding techniques, including the introduction of non-conductive powdersto silicone resin during the molding of the diaphragm to increase itsdurometer hardness.

The disclosed embodiments are representative of presently preferredforms of the invention, but are intended to be illustrative rather thandefinitive thereof. The invention is defined in the claims.

We claim:

1. A thermally sensitive switch comprising a heat conductive body havingfirst and second chambers therein and a tapered throat establishingcommunication between said chambers; normally unconnected, electricallyconductive means in one of said chambers; resilient, compressivelydeformable switching means occupying said one of said chambers andresponsive to the application of compressive force thereto toelectrically connect said conductive means; thermally expansible andcontractile means occupying the other of said chambers and operable inresponse to a rise in its temperature to expand in a direction towardsaid one of said chambers; and resilient, deformable force transmittingmeans occupying said tapered throat and operable in response toexpansion of said expansible and contractile means to engage andtransmit compressive force to said switching means, the taper of saidthroat effecting compression of said force transmitting means andstorage therein of spring energy sufficient to effect withdrawal of saidforce transmitting means from compressive engagement with said switchingmeans in response to contraction of said expansible and contractilemeans.

2. A switch according to claim 1 wherein said switching means isnormally non-conductive and is rendered conductive in response to theapplication of compressive force thereto.

3. A switch according to claim 1 wherein said other of said chambers hasa deformable wall.

4. A switch according to claim'l wherein said electrically conductivemeans comprises a portion of said body communicating with said chamberand a conductor insulated from said body and communicating with saidchamber.

5. A switch according to claim 1 wherein said electrically conductivemeans comprises a pair of conductors insulated from each other and fromsaid body and communicating with said chamber.

6. A switch according to claim 1 wherein said switching means comprisesa pad of non-conductive, elastomeric material containing a plurality ofdiscrete, electrically conductive particles.

7. A switch according to claim 6 wherein said particles are present insuch quantity and are of such size that said pad is non-conductive inthe absence of the application of compressive force thereto.

8. A switch according to claim 1 wherein said force transmitting meansseals said other chamber.

9. A thermally sensitive switch comprising a heat conductive bodydefining a chamber having a lesser cross-sectional area at one end thanat its other end, and a throat establishing communication between thedifferent cross-sectional areas of said chamber and tapering in thedirection of said one end of said chamber; spaced apart electricallyconductive means communicating with said chamber at said one endthereof; resilient, compressively deformable switching means located atsaid one end of said chamber and being of less cross-sectional area thanthat of said one end of said chamber, said switching means beinfdeformable in response to the application of compressive force theretoto correspond substantially in cross-sectional area to thecross-sectional area of said one end of said chamber and bridge saidconductive means; thermally expansible and contractile means occupyingthe other end of said chamber and operable in response to a rise intemperat ure to expand in a direction toward said switching means; andresilient, deformable force transmitting means interposed between saidswitching means and said thermally expansible and contractile means andoccupying said tapered throat, said force transmitting means beingdeformable toward said switching means in response to expansion of saidexpansible and contractile means for transmitting said compressive forceto said switching means, the taper of said throat effecting compressionof said force transmitting means and storage therein of spring energysufficient to effect withdrawal of said force transmitting means fromcompressive engagement with said switching means in response tocontraction of said expansible and contractile means.

10. A switch according to claim 9 wherein said conductive meanscomprises a conductor in engagement with said switching means, and aportion of said body adjacent said one end of said chamber.

11. A switch according to claim 9 wherein said conductive meanscomprises a pair of conductors extending into said chamber at said oneend thereof and straddling said switching means.

12. A switch according to claim 9 wherein said switching means issandwiched between electrically insulating members.

13. A switch according to claim 9 wherein said switching means isnormally non-conductive and is rendered conductive in response to theapplication of compressive force thereto.

14. A switch according to claim 9 wherein said switching means comprisesa pad of non-conductive, elastomeric material containing a plurality ofdiscrete, electrically conductive particles.

15. A switch according to claim 14 wherein said particles are present insuch quantity and are of such size that said pad is non-conductive inthe absence of the application of compressive force thereto.

16. A switch according to claim 9 wherein said force transmitting meansis composed of a resilient material having a normal hardness andcontaining a substance in an amount sufficient to vary said normalhardness.

1. A thermally sensitive switch comprising a heat conductive body havingfirst and second chambers therein and a tapered throat establishingcommunication between said chambers; normally unconnected, electricallyconductive means in one of said chambers; resilient, compressivelydeformable switching means occupying said one of said chambers andresponsive to the application of compressive force thereto toelectrically connect said conductive means; thermally expansible andcontractile means occupying the other of said chambers and operable inresponse to a rise in its temperature to expand in a direction towardsaid one of said chambers; and resilient, deformable force transmittingmeans occupying said tapered throat and operable in response toexpansion of said expansible and contractile means to engage andtransmit compressive force to said switching means, the taper of saidthroat effecting compression of said force transmitting means andstorage therein of spring energy sufficient to effect withdrawal of saidforce transmitting means from compressive engagement with said switchingmeans in response to contraction of said expansible and contractilemeans.
 2. A switch according to claim 1 wherein said switching means isnormally non-conductive and is rendered conductive in response to theapplication of compressive force thereto.
 3. A switch according to claim1 wherein said other of said chambers has a deformable wall.
 4. A switchaccording to claim 1 wherein said electrically conductive meanscomprises a portion of said body communicating with said chamber and aconductor insulated from said body and communicating with said chamber.5. A switch according to claim 1 wherein said electrically conductivemeans comprises a pair of conductors insulated from each other and fromsaid body and communicating with said chamber.
 6. A switch according toclaim 1 wherein said switching means comprises a pad of non-conductive,elastomeric material containing a plurality of discrete, electricallyconductive particles.
 7. A switch according to claim 6 wherein saidparticles are present in such quantity and are of such size that saidpad is non-conductive in the absence of the application of compressiveforce thereto.
 8. A switch according to claim 1 wherein said forcetransmitting means seals said other chamber.
 9. A thermally sensitiveswitch comprising a heat conductive body defining a chamber having alesser cross-sectional area at one end than at its other end, and athroat establishing communication between the different cross-sectionalareas of said chamber and tapering in the direction of said one end ofsaid chamber; spaced apart electrically conductive means communicatingwith said chamber at said one end thereof; resilient, compressivelydeformable switching means located at said one end of said chamber andbeing of less cross-sectional area than that of said one end of saidchamber, said switching means beinf deformable in response to theapplication of compressive force thereto to correspond substantially incross-sectional area to the cross-sectional area of said one end of saidchamber and bridge said conductive means; thermally expansible andcontractile means occupying the other end of said chamber and operablein response to a rise in temperature to expand in a direction towardsaid switching means; and resilient, deformable force transmitting meansinterposed between said switching means and said thermally expansibleand contractile means and occupyiNg said tapered throat, said forcetransmitting means being deformable toward said switching means inresponse to expansion of said expansible and contractile means fortransmitting said compressive force to said switching means, the taperof said throat effecting compression of said force transmitting meansand storage therein of spring energy sufficient to effect withdrawal ofsaid force transmitting means from compressive engagement with saidswitching means in response to contraction of said expansible andcontractile means.
 10. A switch according to claim 9 wherein saidconductive means comprises a conductor in engagement with said switchingmeans, and a portion of said body adjacent said one end of said chamber.11. A switch according to claim 9 wherein said conductive meanscomprises a pair of conductors extending into said chamber at said oneend thereof and straddling said switching means.
 12. A switch accordingto claim 9 wherein said switching means is sandwiched betweenelectrically insulating members.
 13. A switch according to claim 9wherein said switching means is normally non-conductive and is renderedconductive in response to the application of compressive force thereto.14. A switch according to claim 9 wherein said switching means comprisesa pad of non-conductive, elastomeric material containing a plurality ofdiscrete, electrically conductive particles.
 15. A switch according toclaim 14 wherein said particles are present in such quantity and are ofsuch size that said pad is non-conductive in the absence of theapplication of compressive force thereto.
 16. A switch according toclaim 9 wherein said force transmitting means is composed of a resilientmaterial having a normal hardness and containing a substance in anamount sufficient to vary said normal hardness.